1. Field of Invention
The present invention relates to a design and manufacturing method for a laser driver circuit. More particularly, the present invention relates to a design and manufacturing method for utilizing a continuous mode laser driver circuit for burst mode application. The method maintains the level of a transmission signal in a suitable position, so that the cross points of the eye diagram of the laser emitting apparatus are maintained at a predetermined level and the bit error rate is decreased.
2. Description of Related Art
In the past, the signal of optical fiber communication is usually transmitted in continuous mode. In continuous mode, no signal is interrupted during the transmission process, the structure of optical fiber communication can only be established as the structure where one receiver connects to one transmitter. The structure is also called a P2P (Point to Point) structure.
However, if FTTX (Fiber To The X, X representing a home, building, curb, . . . etc) is used, since the signal don't need to be transmitted at all the time, thus it will be a waste for the bandwidth of communication and the fiber if using a communication apparatus of continuous mode.
Because of the problems described above, the designer of an optical fiber communication system develops a FSAN (Full Service Access Network) structure for FTTX application. In FSAN, an important concept is P2MP (Point To Multi Points); When a user wants to transmit some information, the user is allowed to transmit the information immediately; After the completion of transmitting the information, the user is allowed to turn the laser off to avoid signal conflicts. Such transmission is so called burst mode, which is different from the continuous mode mentioned above.
For an optical fiber communication system used for burst mode application, a main role is the laser driver chip. For burst mode laser driving, the laser driver circuit must take very short time to turn on and drive a communication laser diode to transmit signal and then turn off the laser diode completely and rapidly when there is no signal input.
In the FSAN standard, there is no clear definition for the signal present time and stop time in burst mode. Therefore, the system integration companies have developed many different solutions. Some of the solutions need a strict timing control. Thus, the laser driver chip is hard to be available and the production cost is increased.
In burst mode optical fiber communication system, when the NZR (Non Return to Zero) data rate is lower than 1.5 G bps, a direct modulation of laser driver can be used. When the NRZ data rate is higher than 1.5 bps, an external modulator is needed. This invention is used in the burst mode optical fiber communication of which the NRZ data rate is lower than 1.5 G.
In direct modulation, a bias current and a modulation current is needed fora laser diode. Usually, a laser driver chip is used to provide the bias current and the modulation current. The laser driver circuit receives the input voltage signal and then converts the input voltage signal into a current strength.
In continuous mode, the laser driver circuit usually has an APC (Average Power Control) function which cooperates with a PD (Photo Detector), so that the average optical output power and cross points of the optical eye diagram are maintained at an optimal level. At present, there are many ICs with the APC or APD function for continuous mode application in market. But only a few specially designed integrated circuits are available for burst mode application. To obtain the best level of optical output power and cross points of the optical eye diagram for burst mode application, these specially designed integrated circuits usually have DAPC (Digital Automatic Power Control) functions which cooperates with a PD (Photo Detector) or external bias capacitors that can be used for bias current switching. The external bias capacitors are used to control the bias current stability of the laser diodes. However, the two integrated circuits mentioned above have the disadvantages of high manufacturing cost and long bias setting time.